The present invention is directed to an active noise and vibration control (ANVC) system. More particularly, the present invention relates to certain improvements in ANVC systems permitting enhancement of control over a range of frequencies including broadband control and optimization of total energy within the system. The present application is related to application Ser. No. 08/347,521, filed Nov. 30, 1994 entitled "Frequency-Focused Actuators for Active Vibration Energy Control Systems".
Various active noise control (ANC) systems have been proposed which generate an inverted-phase signal of comparable frequency and magnitude to the input, or disturbance, signal which combines destructively with the disturbance signal to eliminate or, at least, significantly reduce the noise within a control volume such as, for example, the interior of an aircraft cabin. A broadband actuator, typically a speaker, has to be of significant size to produce the low-frequency vibrations (20-100 Hz) needed for destructive interference making their placement within the cabin problematic. The problem is aggravated by the fact that in order to control the high-frequency vibrations in the range of 100-500 Hz, there needs to be a large number of speakers because of the increased number of modes. Normally, for higher frequencies the control efficiency tends to be localized within one-tenth of a wavelength from the closest error sensor (which is generally a microphone). The placement of actuators is more critical for high-frequency vibrations.
Similar problems arise in active vibration control (AVC) systems with actuators having to be sized to accommodate the low-frequency (typically, high amplitude) vibrations while the number utilized must be determined by the highest frequency for which control is desired. In addition, systems like Fuller (U.S. Pat. No. 4,715,559) which solely employ actuators to control sound energy to cancel tonal noise can actually input large amounts of vibrational energy into the system to accomplish optimum sound reduction at the error microphones. This increased vibrational energy put into the system can have a negative impact on the fatigue life of the structure. Further, optimum passenger comfort is actually arrived at by a compromise solution resulting in a less-than-optimum noise control in favor of avoiding excessive structural vibration.
The present invention solves the problems of the prior art ANVC devices by subdividing the control responsibility of the low (20-100 Hz, for example) frequency from the high-frequency (100-500 Hz) actuators by frequency focusing the respective actuator groups, permitting the physical size, the force capability, and the number of actuators in the respective groups to be optimized for the application. The term "actuator" when used herein shall include both speakers and structural actuators such as inertial shakers and piezoelectric actuators unless otherwise specified. Further, although the term "high-frequency" is used here to contrast it from the low-frequency band described herein, the range of 100-500 Hz is normally regarded as midrange. Finally, the term "vibrational energy" when used herein shall refer to both structural vibrational and audible or sound vibrational energy.
Another aspect of the present invention is a hybrid speaker and structural actuator system which employs these actuators to maximize the respective advantages of each. Elliott et al. (U.S. Pat. No. 5,170,433) infers a system which uses a combination of equal numbers of speakers and inertial actuators to cancel one or more harmonics of a tonal noise signal (FIG. 10). The present invention uses structural actuators to control noise in the low-frequency range (.ltoreq.70 Hz) where the interior noise is directly coupled to the structural vibration. Either microphones or accelerometers could serve as error sensors for the low-frequency actuators. In the high-frequency range where the interior noise is not directly coupled to structural vibration, it is preferred to use speakers to control noise so as not to increase the structural vibrational energy in the compartment while quieting the noise. Microphones should be used as error sensors in the high-frequency range. While microphones may be shared as error sensors for both low- and high-frequency actuators, the accelerometers should be frequency focused for use by only the structural actuators.
It is well known that the number of actuators required for a particular ANVC system is equal to the number of vibrational energy modes participating in the system response. If a particular cabin is, through experimentation, shown to have K vibrational energy modes, then the number of low-frequency actuators M needed to achieve global noise reduction is given by the expression M.gtoreq.K. For high-frequency control, where the number of vibrational energy modes is greater, it is generally impractical to achieve global control due to the large number of actuators needed. For local control, which produces optimum control efficiency within one-tenth of a wavelength of the error sensor, the number of actuators N needed is related to the number of sensors L by the expression N.gtoreq.L/2; that is, the number of actuators must be equal to or greater than one half the number of error sensors employed in the system to produce the desired reduction of sound at each of the error sensors.
The majority of ANC and ANVC systems have tonal-control capability only, that is, they are not able to handle multiple tones and/or background noise. The present invention includes, as one aspect thereof, an ANVC system employing a broadband reference-signal-detecting means producing an output signal indicative of the broadband noise and vibration to be canceled within the cabin, error sensor means for detecting a residual level of vibrational energy within the cabin downstream of said reference signal means, actuator means capable of generating a phase-inverted signal to reduce at least some portions of the broadband vibrational energy within said compartment, and a broadband controller which includes a plurality of adaptive filters for generating broadband, time-domain command signals which activate said actuators to produce the desired control signal(s).
Various other features, advantages and characteristics of the present invention will become apparent after a reading of the following detailed description thereof.